1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element in which a light-emitting layer capable of emitting light by application of an electric field is provided between a pair of electrodes, and also relates to a light-emitting device and a lighting device including such a light-emitting element.
2. Description of the Related Art
Light-emitting elements which include organic compounds as a light-emitting layer and are characterized by thinness, lightweight, fast response, and direct current driving with low voltage are expected to be applied to next-generation flat panel displays. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
It is said that a light-emitting element has the following light emission mechanism: when voltage is applied between a pair of electrodes with a light-emitting layer including a luminous body provided therebetween, electrons injected from the cathode and holes injected from the anode are recombined in an light emission center of the light-emitting layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as excited states, and light emission can probably be obtained through either state. Light emission from the singlet excited state (S*) is called fluorescence, and light emission from the triplet excited state (T*) is called phosphorescence.
In order to improve element characteristics or productivity of such light-emitting elements, improvement of an element structure, development of a material, and the like have been actively carried out. Further, research and development have been extensively conducted on organic EL elements as light-emitting elements, and full-color organic EL elements have been actively developed.
As a way to achieve full-color display, for example, light-emitting layers of pixels are separately deposited. The light-emitting layers are deposited on necessary pixels using a shadow mask. In this case, to reduce cost by reducing the number of steps, Patent Document 1 discloses a structure in which layers except a light-emitting layer, for example, a hole-transport layer, an electron-transport layer, and a cathode are formed to be shared by a plurality of pixels.